The formation of slime often occurs in water systems, such as a cooling water tower, an industrial water supply system, or a paper-making process, resulting from the growth and proliferation of bacteria, fungi, algae, etc. Such proliferation of microorganisms and slime formation bring about serious problems: in paper manufactories, it may give rise to slowdowns in operations and deterioration of products such as change in color of manufactured paper; reduction in the efficiency of heat exchange, which is critical for cooling capacity, in cooling systems may occur; the appearance of ornamental fountains may be damaged; and in cooling towers of buildings, especially, the genus Legionella may be rapidly spread, creating an unsanitary environment.
Oxidizing biocides and non-oxidizing biocides have been typically used to prevent the contamination of water system by microorganisms. Oxidizing biocides, which have oxidizing power, act as a biocidal agent against microorganisms by oxidizing their proteins, while non-oxidizing biocides work by inhibiting metabolism of microorganisms.
Examples of non-oxidizing biocides include isothiazolone, methylenebisisocyanate, glutaraldehyde, and quaternary ammonium. Such non-oxidizing biocides hold their biocidal activity for a longer period of time than oxidizing biocides, but when they are continuously supplied, microorganisms develop resistance to them, thereby lowering their effectiveness.
On the other hand, oxidizing biocides are typically prepared by chlorination and bromination, as represented by the following equations.
ChlorinationCl2+H2O←→HCl+HOClHOCl→←H++OCl−
BrominationHOCl+Br−←→HOBr+Cl−HOBr→←H++OBr−
Of the two methods, the chlorination method is generally used for preparation of oxidizing biocides because it is low in cost. In this method, alkali or alkaline earth metal hypochlorite, such as sodium hypochlorite (NaOCl), is used as a source of HOCl or OCl− to eliminate microorganisms growing in a variety of water systems including cooling water towers of buildings or petroleum plants, bleaching processes of paper-making factories, and swimming pools. However, the oxidizing biocides have a disadvantage in that they are unstable due to their high volatility. There have been efforts to solve these problems. Low volatile and stabilized N-chlorosulfamate can be prepared by reacting unstable hypochlorite with an equal molar ratio of sulfamic acid, as disclosed in U.S. Pat. No. 3,328,294 (hereinafter, referred to as '294 patent), and can be further stabilized along with the presence of a buffering agent capable of maintain pH between 4.5 and 8.5, as disclosed in U.S. Pat. No. 3,767,586 (hereinafter, referred to as '586 patent).
Although such chlorine biocides are inexpensive and can be prepared by a simple process, their disinfection efficiency may be reduced by a high pH or amines, and upon being used in large quantities, they can promote corrosion of metal, components of water systems, as well as releasing a lot of chlorine to atmosphere.
Oxidizing biocides prepared by the bromination method are, even under the condition of high pH or the presence of amines, more effective in controlling microorganism growth than those prepared by the chlorination. However, just like hypochlorite, hypobromite, as a product of the bromination, is also not stable under conventional storage conditions. Thus, to establish the stability of hypobromite during storage, processes for preparing stabilized hypobromite in U.S. Pat. No. 6,037,318 (hereinafter, referred to as '318 patent) have been proposed, the right to which have been transferred to the Procter & Gamble Company, and U.S. patents including U.S. Pat. Nos. 5,683,654; 5,795,487; 5,942,126; and 6,136,205.
As described especially in U.S. Pat. No. 5,795,487 (hereinafter, referred to as '487 patent), alkali or alkaline earth metal hypobromite in the unstable state, which is prepared by mixing alkali or alkaline earth metal hypochlorite and a water soluble bromide ion source, can be stabilized by employing an alkali metal sulfamate as a stabilizing agent.
Although the stabilized alkali or alkaline earth metal hypobromite prepared by the process of '487 patent shows an excellent biocidal efficiency, the level of total halogen residual which is a measure of an effective ingredient typically drops sharply at an initial phase of the reaction because hypochlorite reacts with a bromide ion source. Thus, with the passage of time, the biocidal efficiency of the hypobromite is rapidly reduced, thus requiring a continuous supply of a great quantity of expensive hypobromite, to maintain biocidal efficiency in water systems.
'318 patent discloses a stabilized hypobromite solution prepared by reacting an aqueous solution of alkali or alkaline earth metal hypochlorite with sulfamic acid as a stabilizing agent at pH below 11, adding to the solution water-soluble bromide ion source, and adjusting pH to about at least 13.
However, the process provided in '318 patent has a disadvantage. Because sulfamic acid used as a stabilizer becomes a strong acid upon being dissolved in water, addition of hypochlorite into a solution of sulfamic acid induces the release of gaseous chlorine, which is an effective ingredient, causing a large reduction in disinfection efficiency, as well as a harsh working environment for preparation of biocides. Moreover, owing to increased temperature by released heat during preparation, as well as the gas generation, pressure is increased. Consequently, the process mentioned in '318 patent is hard to apply in industry. In addition, since the hypochlorite added to the aqueous solution of the stabilizing agent is having a pH not exceeding about 11, the added bromide ion source immediately reacts with hypochlorite to form hypobromite, causing a problem in that its biocidal activity is not maintained for a long period of time.
As disclosed in U.S. Pat. No. 6,270,722 (hereinafter, referred to as '722 patent), stabilized hypobromite can be also prepared by mixing a stabilizing agent and a bromide ion source and adding hypochlorite to the mixture. In this patent, in order to produce a stabilized hypobromite, a stabilizer is, primarily, mixed with a bromide ion source, followed by supplementing with a sodium hypochlorite solution at a pH below about 7 and at below 80° F., and adjusting the pH of the mixture to at least 13 using an alkaline source such as a sodium hydroxide. The resulting product is characterized by an amber color and comprises about 90% of oxidizing bromine compounds, and has a biocidal effect similar to a biocide prepared according to '487 patent (STABREX™, Nalco Chemical Company), indicating that the resulting product is, like the commercially available biocide, a stabilized hypobromite, and when sodium hypochlorite is added into a mixture of bromide ion and a stabilizer having a pH below 7, bromide ion immediately reacts with sodium hypochlorite. Although the biocide prepared according to '722 patent is as effective against microorganisms as one manufactured according to '487 patent, the level of total halogen residual which is a measure of an effective ingredient is rapidly consumed, causing sharp reduction in the biocide's antibacterial activity with the passage of time. Therefore, it is required for a great quantity of the biocide to be supplied to maintain its bactericidal activity for a long time.
Also, stabilized hypobromite can be produced through prior addition of bromide ion and a stabilizer to periodically supplement sodium hypochlorite in swimming pools, as disclosed in U.S. Pat. No. 6,110,387 (hereinafter, referred to as '387 patent).
'387 patent teaches that the growth of microorganisms in swimming pools can be controlled by first adding bromide ion and a stabilizer and then periodically introducing a suitable chlorine biocide. Generally, since water of a swimming pool is at a neutral pH ranging from 7 to 8, bromide ions previously existing in a swimming pool react with hypochlorite, which is generated from a later-added chlorine oxidant, to form unstable hypobromite, and the unstable hypobromite reacts with pre-existing sulfamic acid, resulting in production of stabilized hypobromite. This method is very useful in water systems with no circulation of water, such as swimming pools, and features biocidal activity as effective as the biocide mentioned in '487 patent, but not in circulating water systems having continuous air contact, such as cooling water towers in plants, because of the large loss of the volatile chlorine biocide.
On the other hand, in order to provide stability of hypobromite during storage and convenience in its use, some methods for preparing it in a tablet form are mentioned in U.S. Pat. Nos. 4,557,756, 4,557,926, and 5,688,515 (hereinafter, referred to as '515 patent), and it is now commercially available as Towerbrom™. Especially, '515 patent describes water stable tablets for disinfecting recirculating water systems, comprising chlorinated isocyanurates, sodium bromide, and a stabilizer, in which the stabilizer is compatible, i.e., unreactive, with chlorinated isocyanurates, giving structural integrity to the tablets upon exposure to water and allowing the tablets to dissolve in water at relatively uniform and commercially acceptable rates, and capable of binding active chlorine generated by chlorinated isocyanurates, producing chlorinated stabilizer capable of storing active chlorine. Upon the tablets being immersed in water, the chlorinated stabilizer is able to slowly release active chlorine thanks to its lower solubility in water than chlorinated isocyanurates, and the active chlorine reacts with sodium bromide to generate hypobromite.
The method mentioned in '515 patent provides a biocide with an excellent efficiency. However, it is not convenient and economical in the views of requiring a specific supply equipment to maintain a constant concentration of active chlorine, and using an expensive chlorine oxidant as an active chlorine source.